1. Field of the Invention
This invention relates to an improved closure for flexible bags such as thermoplastic bags comprising polyethylene. Related applications assigned to the same assignee as this application include the following copending and concurrently filed applications: "Adhesive Channel Closure for Flexible Bags," Ser. No. 335,798, filed Dec. 30, 1981; "Laminated Pressure Sensitive Adhesive Systems for Use in Plastic Bags," Ser. No. 335,799, filed Dec. 30, 1981; "Manufacturing Process for Channel Seal," Ser. No. 365,814, filed Apr. 5, 1982; and "Adhesive Bag Closure That Opens Easily By Hand But Resists Opening By Contents", Ser. No. 335,800, filed Dec. 30, 1981, the last two applications being incorporated by reference herein.
2. Brief Description of the Prior Art
A variety of closures have been developed for use with flexible bags such as thermoplastic bags made out of polyethylene film. These closures included the so-called profile bags where the closure comprises one or more sets of mating channels. One example of a profile closure is the Zip-loc storage bag as found in U.S. Pat. No. Re. 28,969 to Naito. U.S. Pat. No. 4,186,786 to Kirkpatrick shows colored channels so that the user may more easily detect complete occlusion of profile bag openings.
Other types of bag closures use one or more adhesive strips. See, for example, U.S. Pat. No. 3,670,876 to Davis. A major problem with adhesive closures has been that the adhesive must be able to form a good resealable closure when contacted with the other surface forming the bag opening, and yet the adhesive must be storable in a package with other bags without destructively adhering to the other bags.
An attempt to combine a profile closure with an adhesive may be seen in U.S. Pat. No. 3,339,606 to Kugler. The releasable closure in the Kugler patent comprises a tongue on one member and a groove on the other member where the tongue is of a thickness less than the width of the groove and wherein a releasable pressure sensitive adhesive is provided to keep the tongue within the groove. This structure, however, still requires registration of mating channels.
Attempts have also been made to protect the adhesive strip until the bag is used. In U.S. Pat. No. 3,420,433 to Bostwick a closure flap having an adhesive applied thereto is folded or overflapped upon itself to form a protective enclosure for the adhesive prior to the use of the bag.
In U.S. Pat. No. 3,990,627 to Olson (and assigned to the same assignee as this application), a Z-fold adhesive closure for bags is disclosed wherein the adhesive strip is covered by the upper portion of the bag's front wall until ready for use. It is difficult, however, to find an adhesive which is easy to apply, which is strong enough to form an effective seal upon closure, but which does not cause undesirable problems by sticking too strongly to the front wall closed over the adhesive in a stored position. An alternate embodiment of the Olson patent uses a release tape permanently secured to the inner surface of the bag's front wall flap, whereby when the flap is folded back upon itself, the release tape is brought into protective covering relationship with the adhesive strip located on the inner surface of the frontwall flap. The use of a release layer is not desirable for plastic bag closures because of the expense and difficulty in manufacture and because release material may transfer to the adhesive layer.
For Z-fold type bags the problems in constructing a resealable closure capable of forming a satisfactory seal are compounded by the intimate contact between the adhesive strip and the protective strip or overflap. It is desirable that the initial exposure of the adhesive strip be accomplished with as little force as possible, yet the adhesive must be sufficiently aggressive to hold adequately when it is used to close the bag. In practice this is difficult to accomplish because pressure-sensitive adhesives, especially those of the hot-melt type, typically increase in adhesion as they are left in contact for a long period of time. Since the Z-fold is left in contact for a very long time (from the time the bag is manufactured until it is used), normally the force to expose the adhesive strip for the first time is the maximum for a given adhesive material. One way in which this problem has been reduced in the past has been by the use of corona discharge treatment of the film only in the areas to be contacted by the adhesive for final closure. Since adhesives stick more to treated surfaces, this gives a preferentially greater adhesion in final use, allowing a lower tack adhesive material to be used so that the Z-fold is easier to open for initial exposure. This type of closure is not fully satisfactory because only a small differential in adhesion is achieved by such treatment.
Thus, it is an object of the present invention to provide an improved Z-fold closure which minimizes the contact between an adhesive strip and a protective strip. It is a further object of this invention to provide an improved Z-fold closure which may utilize a stronger adhesive. It is yet another object of this invention to provide an improved Z-fold closure which maintains separability of its adhesive strip and protective strip even during prolonged storage. These and other objects of the invention will be apparent from the following explanation.